Optical diffuser film and light assembly

ABSTRACT

A light assembly comprises a light source, an optically transmissive self- supporting substrate, and coupled with but unattached to the substrate, a voided high Tg semi-crystalline polymeric optical diffuser film that shrinks less than 1% as a result of thermal shrinkage testing.

FIELD OF THE INVENTION

The invention relates to optical displays, and more particularly toliquid crystal displays (LCDs) that may be used in LCD monitors and LCDtelevisions.

BACKGROUND

Liquid crystal displays (LCDs) are optical displays used in devices suchas laptop computers, hand-held calculators, digital watches andtelevisions. Some LCDs include a light source that is located to theside of the display, with a light guide positioned to guide the lightfrom the light source to the back of the LCD panel. Other LCDs, forexample some LCD monitors and LCD televisions (LCD-TVs) are directlyilluminated using a number of light sources positioned behind the LCDpanel. This arrangement is increasingly common with larger displays,because the light power requirements, to achieve a certain level ofdisplay brightness, increase with the square of the display size,whereas the available real estate for locating light sources along theside of the display only increases linearly with display size. Inaddition, some LCD applications, such as LCD-TVs, require that thedisplay be bright enough to be viewed from a greater distance than otherapplications, and the viewing angle requirements for LCD-TVs aregenerally different from those for LCD monitors and hand-held devices.

Some LCD monitors and most LCD-TVs are commonly illuminated from behindby a number of cold cathode fluorescent lamps (CCFLs). These lightsources are linear and stretch across the full width of the display,with the result that the back of the display is illuminated by a seriesof bright stripes separated by darker regions. Such an illuminationprofile is not desirable, and so a diffuser plate is used to smooth theillumination profile at the back of the LCD device.

Currently, LCD-TV diffuser plates employ a polymeric matrix ofpolymethyl methacrylate (PMMA) with a variety of dispersed phases thatinclude glass, polystyrene beads, and CaCO₃ particles. These platesoften deform or warp after exposure to the elevated humidity and hightemperature caused by the lamps. In addition, the diffusion platesrequire customized extrusion compounding to distribute the diffusingparticles uniformly throughout the polymer matrix, which furtherincreases costs.

A previous disclosure, U.S. Pat. Publication No. 2006/0082699 describesone approach to reducing the cost of diffusion plates by laminatingseparate layers of a self-supporting substrate and an optically diffusefilm. Although this solution is novel the need to use adhesives tolaminate these layers together results in reduced efficiency of thesystem by adding light absorption materials. Also the additionalprocessing cost to laminate the layers together is self-defeating. Also,this previous disclosure does not teach the materials and structure foran unattached diffuser film. It is desirable to have an unattacheddiffuser film, which must have dimensional stability as well as highoptical transmission while maintaining a high level of lightuniformization. Further, it is desirable for such a diffuser to haveadditional heat insulation value to reduce the heat gain from the lightsources to the LC layer above the diffuser. Voiding is a well-knownmeans to achieve both the optical requirements and the insulationrequirements of the diffuser. A thin diffuser is also desirable asmanufacturers are constantly looking for means to thin the profile ofLCD screens. Producing a thin voided film that meets these requirementsis very challenging as thin voided films are highly prone to shrinkageunder elevated temperatures. Therefore, an object of the presentinvention is to provide a voided polymeric optical diffuser film whichcan be placed adjacent to an optically transmissive self-supportingsubstrate, unattached to said substrate, to provide the opticalsmoothing function of previous plate diffusers at a very low cost. Theoptical diffuser film is unique in that it provides a high level ofoptical function and meets dimensional stability requirements underspecified thermal testing even at low thicknesses.

SUMMARY OF THE INVENTION

One embodiment of this invention is a light assembly containing a voidedhigh Tg semi-crystalline polymeric optical diffuser film with shrinkageof less than 1% as a result of thermal shrinkage testing. This film isuseful in replacing the optical function of diffuser plates typicallyused today in backlit LCD displays.

Another embodiment of the invention is directed to a liquid crystaldisplay (LCD) unit that has a light source and an LCD panel thatincludes an upper plate, a lower plate and a liquid crystal layerdisposed between the upper and lower plates. The lower plate faces thelight source, and includes an absorbing polarizer. An arrangement oflight management layers is disposed between the light source and the LCDpanel so that the light source illuminates the LCD panel through thearrangement of light management layers. The arrangement of lightmanagement layers includes an arrangement of light management films andan optically transmissive self-supporting substrate. The arrangement oflight management films comprises at least a first voided polymericoptical diffuser film. The arrangement of light management filmsoptionally comprises other optical layers. Other optical layers mayinclude a bead coated collimation film, a light directing film and areflective polarizer.

Another embodiment of the invention is directed to a liquid crystaldisplay (LCD) unit that has a light source and an LCD panel thatincludes an upper plate, a lower plate, and a liquid crystal layerdisposed between the upper and lower plates. The lower plate faces thelight source, and includes an absorbing polarizer. An arrangement oflight management layers is disposed between the light source and the LCDpanel so that the light source illuminates the LCD panel through thearrangement of light management layers. The arrangement of lightmanagement layers includes an arrangement of light management films andan optically transmissive self-supporting substrate. The arrangement oflight management films comprises at least a first voided polymericoptical diffuser film and comprising a structured surface to control thedirection of light rays transmitted through the film. The arrangement oflight management films optionally comprises other optical layers. Otheroptical layers may include a light directing film and a reflectivepolarizer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates a typical back-lit liquid crystaldisplay device that uses a diffuser plate;

FIG. 2 schematically illustrates an arrangement of light managementlayers that is capable of using an optically transmissiveself-supporting substrate and a voided polymeric optical diffuser filmaccording to principles of the present invention;

FIG. 3 schematically illustrates an arrangement of light managementlayers that is capable of using an optically transmissiveself-supporting substrate and a voided polymeric optical diffuser filmwith a structured surface to control the direction of light raystransmitted through the film according to principles of the presentinvention.

FIG. 4 shows the testing apparatus useful for the invention.

FIG. 5 shows in graphical form the transmission of the tested samples.

FIG. 6 is a graph showing optical uniformity of one of the samples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is applicable to light assemblies and particularlyliquid crystal displays (LCDs, or LC displays), and is particularlyapplicable to LCDs that are directly illuminated from behind, forexample as are used in LCD monitors and LCD televisions (LCD-TVs).

The diffuser plates currently used in LCD-TVs are based on a polymericmatrix, for example polymethyl methacrylate (PMMA), polycarbonate (PC),or cyclo-olefins, formed as a rigid sheet. The sheet contains diffusingparticles, for example, organic particles, inorganic particles or voids(bubbles). These plates often deform or warp after exposure to theelevated temperatures of the light sources used to illuminate thedisplay. These plates also are more expensive to manufacture and toassemble in the final display device.

The invention is directed to a directly illuminated LCD device that hasan arrangement of light management layers positioned between the LCDpanel itself and the light source. The arrangement of light managementlayers includes an optically transmissive self-supporting organic orinorganic substrate and a voided polymeric optical diffuser filmpossessing a specific transmission and haze level placed directlyadjacent to one side of the substrate, but unattached to said substrate.The transmission and haze levels of each component are designed toprovide a direct-lit LC display whose brightness is relatively uniformacross the display.

The optically transmissive self-supporting organic or inorganicsubstrate S of the present invention are simple to manufacture and arecommercially available as a commodity item. Voided polymeric opticaldiffuser films of the present invention are simple to manufacture andprovide a high degree of flexibility in the materials and processes usedin manufacturing. In the present invention, the structural and opticalrequirements are separated: the substrate provides the structuralperformance and the unattached diffusing layer, provides the opticalperformance. By separating these functions, the cost advantages of usingcommon transparent materials and common diffuser sheets can beexploited, to reduce overall costs. By not attaching the substrate andthe diffuser film a high level of optical performance and a lowmanufacturing cost is realized. This also permits the introduction ofwarp resistant plates, for example glass plates, at low cost. Inaddition, it is easier to control the diffusion properties moreprecisely when the diffuser is contained in a film rather than asubstrate. By using a voided diffuser film a higher level of insulationcan be provided at any given thickness of the diffuser. By beingunattached, however, the diffuser must meet thermal shrinkagerequirements of other optical films in the arrangement.

A schematic exploded view of an exemplary embodiment of a direct-lit LCdisplay device 100 is presented in FIG. 1. Such a display device 100 maybe used, for example, in an LCD monitor or LCD-TV. The display device100 is based on the use of a front panel assembly 130, comprising a LCpanel 140, which typically comprises a layer of LC 136 disposed betweenpanel plates 134. The plates 134 are often formed of glass, and mayinclude electrode structures and alignment layers on their innersurfaces for controlling the orientation of the liquid crystals in theLC layer 136. The electrode structures are commonly arranged so as todefine LC panel pixels, areas of the LC layer where the orientation ofthe liquid crystals can be controlled independently of adjacent areas. Acolor filter may also be included with one or more of the plates 134 forimposing color on the image displayed.

An upper absorbing polarizer 138 is positioned above the LC layer 136and a lower absorbing polarizer 132 is positioned below the LC layer136. The absorbing polarizers 138, 132 and the LC panel 140 incombination control the transmission of light from the backlight 110through the display 100 to the viewer. In some LC displays, theabsorbing polarizers 138, 132 may be arranged with their transmissionaxes perpendicular. When a pixel of the LC layer 136 is not activated,it may not change the polarization of light passing there through.Accordingly, light that passes through the lower absorbing polarizer 132is absorbed by the upper absorbing polarizer 138, when the absorbingpolarizers 138, 132 are aligned perpendicularly. When the pixel isactivated, on the other, hand, the polarization of the light passingthere through is rotated, so that at least some of the light that istransmitted through the lower absorbing polarizer 132 is alsotransmitted through the upper absorbing polarizer 138. Selectiveactivation of the different pixels of the LC layer 136, for example by acontroller 150, results in the light passing out of the display atcertain desired locations, thus forming an image seen by the viewer. Thecontroller may include, for example, a computer or a televisioncontroller that receives and displays television images. One or moreoptional layers 139 may be provided over the upper absorbing polarizer138, for example to provide mechanical and/or environmental protectionto the display surface. In one exemplary embodiment, the layer 139 mayinclude a hardcoat over the absorbing polarizer 138.

It will be appreciated that some type of LC displays may operate in amanner different from that described above. For example, the absorbingpolarizers may be aligned parallel and the LC panel may rotate thepolarization of the light when in an unactivated state. Regardless, thebasic structure of such displays remains similar to that describedabove.

The backlight 110 includes a number of light sources 114 that generatethe light that illuminates the LC panel 130. The light sources 114 usedin a LCD-TV or LCD monitor are often linear, cold cathode, fluorescenttubes that extend across the display device 100. Other types of lightsources may be used, however, such as filament or arc lamps, lightemitting diodes (LEDs), flat fluorescent panels or external fluorescentlamps. This list of light sources is not intended to be limiting orexhaustive, but only exemplary.

The backlight 110 may also include a reflector 112 for reflecting lightpropagating downwards from the light sources 114, in a direction awayfrom the LC panel 140. The reflector 112 may also be useful forrecycling light within the display device 100, as is explained below.The reflector 112 may be a specular reflector or may be a diffusereflector. One example of a specular reflector that may be used as thereflector 112 is Vikuiti® Enhanced Specular Reflection (ESR) filmavailable from 3M Company, St. Paul, Mn. Examples of suitable diffusereflectors include polymers, such as polyethylene terephthalate (PET),polycarbonate (PC), polypropylene, polystyrene and the like, loaded withdiffusely reflective particles, such as titanium dioxide, bariumsulphate, calcium carbonate and the like.

An arrangement 120 of light management layers is positioned between thebacklight 110 and the front panel assembly 130. The light managementlayers affect the light propagating from backlight 110 so as to improvethe operation of the display device 100 . For example, the arrangement120 of light management layers may include a diffuser plate 122. Thediffuser plate 122 is used to diffuse the light received from the lightsources, which results in an increase in the uniformity of theillumination light incident on the LC panel 140. Consequently, thisresults in an image perceived by the viewer that is more uniformlybright.

The arrangement 120 of light management layers may also include areflective polarizer 128. The light sources 114 typically produceunpolarized light but the lower absorbing polarizer 132 only transmits asingle polarization state, and so about half of the light generated bythe light sources 114 is not transmitted through to the LC layer 136.The reflecting polarizer 128, however, may be used to reflect the lightthat would otherwise be absorbed in the lower absorbing polarizer, andso this light may be recycled by reflection between the reflectingpolarizer 128 and the reflector 112. At least some of the lightreflected by the reflecting polarizer 128 may be depolarized, andsubsequently returned to the reflecting polarizer 128 in a polarizationstate that is transmitted through the reflecting polarizer 128 and thelower absorbing polarizer 132 to the LC layer 136. In this manner, thereflecting polarizer 128 may be used to increase the fraction of lightemitted by the light sources 114 that reaches the LC layer 136, and sothe image produced by the display device 100 is brighter.

Any suitable type of reflective polarizer may be used, for example,multilayer optical film (MOF) reflective polarizers; diffuselyreflective polarizing film (DRPF), such as continuous/disperse phasepolarizers, wire grid reflective polarizers or cholesteric reflectivepolarizers.

The arrangement 120 of light management layers may also include a lightdirecting film 126. A light directing film is one that includes asurface structure that redirects off-axis light in a direction closer tothe axis of the display. This increases the amount of light propagatingon-axis through the LC layer 136, thus increasing the brightness of theimage seen by the viewer. One example is a prismatic light directingfilm, which has a number of prismatic ridges that redirect theillumination light, through refraction and reflection.

The arrangement 120 of light management layers may also include a lightcollimating diffuser film 124. A light collimating diffuser film istypically a polyester sheet coated with polymertic microbeads and abinder and also helps to re-direct off-axis light in a direction closerto the axis of the display.

Unlike diffuser plates used in conventional LCD-TVs, the presentinvention uses an arrangement of light management layers that haveseparate structural and diffusing members. An optically transmissiveself-supporting substrate and an unattached voided polymeric opticaldiffuser film perform these functions, respectively. One exemplaryembodiment of the present invention is schematically illustrated in FIG.2. The arrangement of light management layers 200 includes an opticallytransmissive self-supporting substrate 212 and a voided polymericoptical diffuser film 214 adjacent to but un-attached to the substrate.Other optical films can be added to the arrangement of light managementlayers above the voided polymeric optical diffuser film 214. These otheroptical films may include a bead coated light collimation film 215, aprismatic light directing film 216, and a reflective polarizer 218.

The substrate 212 is a sheet of material that, like that of the platediffuser in conventional back lights, is self-supporting, and is used toprovide support to the layers above in the light management arrangement.Self-Supporting is thus defined as bending insignificantly(less than1/180 of its longest dimension) under its own weight even with theadditional weight of other layers in the arrangement. The substrate 212may be, for example, up to a few mm thick, depending on the size of thedisplay. For example, in one exemplary embodiment, a 30″ LCD-TV has a 2mm thick bulk diffuser plate. In another exemplary embodiment, a 40″LCD-TV has a 3 mm thick bulk diffuser plate.

The substrate 212 may be made of any material that is substantiallytransparent to visible light, for example, organic or inorganicmaterials, including glasses and polymers. Suitable glasses includefloat glasses, i.e. glasses made using a float process, or LCD qualityglasses, referred as LCD glass, whose characteristic properties, such asthickness and purity, are better controlled than float glass. Oneapproach to forming LCD glass is to form the glass between rollers.

The substrate 212, the diffuser film 214, and one or more other lightmanagement layers may be included in a light management arrangementdisposed between the backlight and the LCD panel. The substrate 212provides a stable structure for supporting the light managementarrangement. The substrate 212 is less prone to warping thanconventional diffuser plate systems, particularly if the supportingsubstrate 212 is formed of a warp-resistant material such as glass.

Suitable polymer materials used to make the substrate 212 may beamorphous or semi-crystalline, and may include homopolymer, copolymer orblends thereof. Example polymer materials include, but are not limitedto, amorphous polymers such as poly(carbonate) (PC); poly(styrene) (PS);acrylates, for example acrylic sheets as supplied under the ACRYLITE®brand by Cyro Industries, Rockaway, N.J.; acrylic copolymers such asisooctyl acrylate/acrylic acid; poly(methylmethacrylate) (PMMA); PMMAcopolymers; cycloolefins; cylcoolefin copolymers; acrylonitrilebutadiene styrene (ABS); styrene acrylonitrile copolymers (SAN);epoxies; poly(vinylcyclohexane); PMMA/poly(vinylfluoride) blends;atactic poly(propylene); poly(phenylene oxide) alloys; styrenic blockcopolymers; polyimide; polysulfone; poly(vinyl chloride); poly(dimethylsiloxane) (PDMS); polyurethanes; poly(carbonate)/aliphatic PET blends;and semicrystalline polymers such as poly(ethylene); poly(propylene);poly(ethylene terephthalate) (PET); poly(ethylene naphthalate)(PEN);polyamide; ionomers; vinyl acetate/polyethylene copolymers; celluloseacetate; cellulose acetate butyrate; fluoropolymers;poly(styrene)-poly(ethylene) copolymers; and PET and PEN copolymers.

Exemplary embodiments of the voided polymeric optical diffuser film 214include a high Tg (glass transition temperature above 80° C.)semi-crystalline polymer matrix containing voids and void initiatingparticles. A high Tg semi-crystalline polymer matrix is preferred as itmay be substantially transparent to visible light, can be readilystretch voided, and can possess dimensional stability having a shrinkageof less than 1.0% after being tested at elevated temperatures up to85C(this is the condition typically required by films in direct backlitLCD's). Preferable polymers to meet all these criteria are polyestersand their copolymers. Most preferred are poly(ethylene terephthalate)(PET); poly(ethylene naphthalate)(PEN)polyesters and any of theircopolymers. PET is most suitable as it is much lower in cost than PEN.FIG. 5 shows the light transmission of several commercially availablePET resins. Transmission is measured per method ASTM D-1003. Some gradeshave a transmission below 90.5%. It is preferred that PET grades withoptical transmissions above 90.5% are used to limit the amount of lightabsorption by the diffuser film.

The void initiating particles may be any type of particle that isincompatible with the matrix polymer. These particles can be inorganicor organic. Inorganic particles can include any of calcium carbonate,barium sulfate, titanium dioxide, or any other inorganic compound thatcan be melt blended into a polymer. Typical organic void initiatingparticles are polymers that are immiscible with the matrix polymer.These are preferred as resin pellets of these immiscible polymers can besimply dry blended with the resin pellets of the matrix polymer andextruded together to form a cast film. Inorganic particles require apre-mixing or melt compounding, which adds processing cost. Preferredorganic void initiating particles are polyolefins. Most preferred ispolypropylene. The void initiating particles should be added so as toproduce enough diffusivity to function as a diffuser yet not be soopaque that the optical luminance of the LCD display is significantlyreduced. Preferred loadings of the void initiating particles are 3 to 25wt % of the entire film. The most preferred loadings are 10 to 20 wt %.%. For optimal diffuser optical performance the void initiator loadingmultiplied by the thickness, in um, of the voided layer after stretchingshould be maintained in a range between 750 and 1500. Preferably thisrange is between 950 and 1350.

The voided polymeric optical diffuser 214 is preferably produced by aprocess of dry blending the matrix polymer and an immiscible polymeradditive. Blending may be accomplished by mixing finely divided, e.g.powdered or granular, matrix polymer and polymeric additive and,thoroughly mixing them together, e.g. by tumbling them. The resultingmixture is then fed to the film forming extruder. Blended matrix polymerand immiscible polymeric additive which has been extruded and, e.g.reduced to a granulated form, can be successfully re-extruded into avoided polymeric optical diffuser. It is thus possible to re-feed scrapfilm, e.g. as edge trimmings, through the process. Alternatively,blending may be effected by combining melt streams of matrix polymer andthe immiscible polymer additive just prior to extrusion. If thepolymeric additive is added to the polymerization vessel in which thematrix polymer is produced, it has been found that voiding and hencediffusivity is not developed during stretching. This is thought to be onaccount of some form of chemical or physical bonding which may arisebetween the additive and matrix polymer during thermal processing.

The extrusion, quenching and stretching of the voided polymeric opticaldiffuser film may be effected by any process which is known in the artfor producing oriented film, e.g. by a flat film process or a bubble ortubular process. The flat film process is preferred for making voidedpolymeric optical diffuser according to this invention and involvesextruding the blend through a slit die and rapidly quenching theextruded web upon a chilled casting drum so that the matrix polymercomponent of the film is quenched into the amorphous state. The filmbase is then biaxially oriented by stretching in mutually perpendiculardirections at a temperature above the glass-rubber transitiontemperature of the matrix polymer. Generally the film is stretched inone direction first and then in the second direction although stretchingmay be effected in both directions simultaneously if desired. In atypical process the film is stretched firstly in the direction ofextrusion over a set of rotating rollers or between two pairs of niprollers and is then stretched in the direction transverse thereto bymeans of a tenter apparatus. The film may be stretched in each directionto 2.5 to 5.0 times its original dimension in each direction ofstretching. Upon stretching voids initiate around the void initiatingparticles. The higher the concentration of void initiating particle thehigher the degree of void volume that is produced. The stretching alsoenhances the degree of crystallinity of the high Tg polymer matrix ofthe film thus making the film less prone to shrinking under testconditions. The final stretched thickness of the film is preferably inthe 1.0 to 10.0 mil thickness range. The most preferred thickness rangeis between 2.0 and 6.0 mils. This is significantly thinner than theoptically transmissive self-supporting substrate and together theirtotal thickness can be maintained in the range of that of the currentlyused plate diffusers.

After the film has been stretched and avoided polymeric optical diffuserfilm formed, it is heat set by heating to a temperature sufficient tocrystallize the matrix polymer whilst restraining the voided polymericoptical diffuser against retraction in both directions of stretching.This process enables the film to meet shrinkage requirements of lessthan 1.0% when tested at temperatures up to 80C. The voiding tends tocollapse as the heat setting temperature is increased and the degree ofcollapse increases as the temperature increases. Hence specular lighttransmission increases with an increase in heat setting temperatures.Whilst heat setting temperatures up to about 230 C can be used withoutdestroying the voids, temperatures between 150 C and 200 C generallyresult in a greater degree of voiding and more efficient duffusivity, aswell as result in low shrinkage after thermal testing.

The voided polymeric optical diffuser film 214 may also include awhitener. Typically whiteners are added at levels much lower than voidinitiators and thus do not contribute to voiding but do improvewhiteness and to some extent diffusivity of the film. Whiteners aretypically inorganic compounds, TiO2 being most preferred. These opticalbrighteners can be added to the film during the resin blending processand can be added via master batch pellets at the appropriate ratio. Theappropriate ratio is that that would let down the concentration of themaster batch pellet with the rest of the matrix resin and voidinitiating resin to a concentration preferably between 0.25 and 5.0 wt%.

The voided polymeric optical diffuser film 214 may also include opticalbrighteners that convert UV light into visible light. Such opticalbrighteners must be chosen from those which are thermally stable and cansurvive the extrusion temperatures used to fabricate the voidedpolymeric optical diffuser film. Preferred optical brighteners comprisebenzoxazolyll-stilbene compounds. The most preferred optical brightenercomprises 2,2′-(1,2-ethenediyldi-4,1-phenylene)bisbenzoxazole. Theseoptical brighteners can be added to the film during the resin blendingprocess and can be added via master batch pellets at the appropriateratio. The appropriate ratio is that that would let down theconcentration of the master batch pellet with the rest of the matrixresin and void initiating resin to a concentration preferably between0.01 and 0.1 wt %. In the most preferred embodiment the opticalbrightener will be added to attain a concentration between 0.02 and 0.05% wt.

The voided polymeric optical diffuser film 214 may also include anantistatic coating to prevent dirt attraction. Anyone of the knownantistatic coatings could be employed.

The voided polymeric optical diffuser film 214 may also be fabricated asa multilayered or coextruded film. Advantages of doing so would be toenable the use of a very thin film yet still meet both optical andthermal stability or shrinkage requirements. Thin films require highloadings of void initiator and thus high voiding to achieve the opticaldiffusion performance of a plate diffuser. At these high levels ofvoiding the film is much less dimensionally stable at elevatedtemperatures. By creating a film with a non-voided layer adjacent to oneor both sides of a voided layer the dimensional stability at elevatedtemperatures can be improved. Such multilayered films are produced thesame as previously discussed except a second extruder is used to meltand pump neat matrix polymer. This neat polymer extrusion flow isdelivered along with the voided layer extrusion flow, previouslydescribed, into a co-extrusion die assembly. A multilayered cast film isthen produced with a layer of neat polymer on one or both sides of thevoided layer. This cast film is then quenched and stretched aspreviously discussed.

The optically transmissive self-supporting substrate 212 or the opticaldiffuser film 214 may be provided with protection from ultraviolet (UV)light, for example by including UV absorbing material or material in oneof the layers that is resistant to the effects of UV light. Suitable UVabsorbing compounds are available commercially, including, e.g.,Cyasorb® UV-1164, available from Cytec Technology Corporation ofWilmington, Del., and Tinuvin® 1577, available from Ciba SpecialtyChemicals of Tarrytown, N.Y.

Other materials may be included in the optically transmissiveself-supporting substrate 212 or the optical diffuser film 214 to reducethe adverse effects of UV light. One example of such a material is ahindered amine light stabilizing composition (HALS). Generally, the mostuseful HALS are those derived from a tetramethyl piperidine, and thosethat can be considered polymeric tertiary amines. Suitable HALScompositions are available commercially, for example, under the“Tinuvin” tradename from Ciba Specialty Chemicals Corporation ofTarrytown, N.Y. One such useful HALS composition is Tinuvin 622.

Another exemplary embodiment of the present invention is schematicallyillustrated in FIG. 3. The arrangement of light management layers 300includes an optically transmissive self-supporting substrate 312 and avoided polymeric optical diffuser film 314 adjacent to but un-attachedto the substrate. Other optical films can be added to the arrangement oflight management layers above the voided polymeric optical diffuser film314. These other optical films may include a, a prismatic lightdirecting film 316, and a reflective polarizer 318.

The voided polymeric optical diffuser film 314 of this arrangement hasbeen fully described previously as that of the voided polymeric opticaldiffuser film 214 of FIG. 2 in the prior arrangement. The voidedpolymeric optical diffuser film 314, however, further comprises astructured surface on the side opposite the optically transmissiveself-supporting substrate 312. The function of the structures on thissurface of the voided polymeric optical diffuser film 314 is to directlight rays which transmit through the film into a more normal directionto the film surface. The structures on this surface are preferablyfinite curved prismatic structures. Such structures have been fullydescribed in U.S. Pat. Publication no. 2006/0092490, which isincorporated by reference. Optically transparent microbeads or anoptical modifying layer can optionally be coated onto these structuresto further help control direction of light rays transmitting through thefilm. Such coatings have been fully described in previously filed U.S.Patent Application No. 60/833,713, which provides a light redirectingfilm comprising a light exit surface bearing (a) optical elements and(b) an optical modification layer containing microbeads and a binderdisposed over the optical elements wherein said light redirecting filmhas an optical gain of at least 1.20. The optical modification layerapplied to the surface of the optical elements allows more incidentlight to pass through the light redirecting film compared to prior artlight redirecting films. It has been found that the optical modificationlayer applied to the surface of the optical elements “frustrates” orreduces the amount of total internal reflection in the light redirectingfilm. The frustration of the total internal reflection of the lightredirecting film results in between 5 and 14% higher light outputcompared to the same light redirecting film without the opticalmodification layer.

Such layers can further control the direction of light transmittingthrough the voided polymeric optical diffuser film.

EXAMPLES

Various samples of voided polymeric optical diffuser films were preparedand their performance in combination with an optically transmissiveself-supporting substrate was compared to commercially availablediffuser films as well as that of the diffuser plate used in acommercially available LCD-TV. The voided polymeric optical diffuserfilms and optically transmissive self-supporting substrates togetherwere tested for brightness and optical uniformity. The voided polymericoptical diffuser films were tested individually for thermal shrinkage aswell. Commercially available foamed or voided films that have beenidentified as potential diffuser layers were also tested in combinationwith optically transmissive self-supporting substrates for brightnessand optical uniformity and where evaluated as unattached films weretested individually for shrinkage as well.

Sample EX-1

PET(#7352 from Eastman Chemicals) was dry blended withPolypropylene(“PP”, Huntsman P4G2Z-159) at 22% by weight and with a 1part PET to 1 part TiO2 concentrate (PET 9663 E0002 from EastmanChemicals) at 1.7% by weight. This blend was then dried in a desiccantdryer at 65° C. for 12 hours.

Cast sheets were extruded using a 2½″ extruder to extrude thePET/PP/TiO2 blend. The 275° C. meltstream was fed into a 7 inch filmextrusion die also heated at 275° C. As the extruded sheet emerged fromthe die, it was cast onto a quenching roll set at 55° C. The PP in thePET matrix dispersed into globules between 10 and 30 um in size duringextrusion. The final dimensions of the continuous cast sheet were 18 cmwide and 305 μm thick. The cast sheet was then stretched at 110° C.first 3.2 times in the X-direction and then 3.4 times in theY-direction. The stretched sheet was then Heat Set at 150° C.

During stretching voids were initiated around the particles of PP thatwere dispersed in the cast sheet. These voids grew during stretching andresulted in significant void volume. The resulting the thickness was 61um.

This film was evaluated optically as an unattached diffuser film incombination with a 2 mm thick plate of float glass.

Sample EX-2

PET(#7352 from Eastman Chemicals) was dry blended withPolypropylene(“PP”, Huntsman P4G2Z-159) at 20% by weight and with a 1part PET to 1 part TiO2 concentrate (PET 9663 E0002 from EastmanChemicals) at 2.0% by weight. This blend was then dried in a desiccantdryer at 65° C. for 12 hours.

Cast sheets were extruded using a 1¼″ extruder to extrude thePET/PP/TiO2 blend. The 275° C. meltstream was fed into a 7-inch filmextrusion die also heated at 275° C. As the extruded sheet emerged fromthe die, it was cast onto a quenching roll set at 55° C. The PP in thePET matrix dispersed into globules between 10 and 30 um in size duringextrusion. The final dimensions of the continuous cast sheet were 18 cmwide and 300 um thick. The cast sheet was then stretched at 110° C.first 3.2 times in the X-direction and then 3.4 times in theY-direction. The stretched sheet was then Heat Set at 150° C.

During stretching voids were initiated around the particles of PP thatwere dispersed in the cast sheet. These voids grew during stretching andresulted in significant void volume. The resulting the thickness was 53μm. This film was evaluated optically as an unattached diffuser film incombination with a 2 mm thick plate of float glass.

Sample EX-3

PET(#7352 from Eastman Chemicals) was dry blended withPolypropylene(“PP”, Huntsman P4G2Z-159) at 20% by weight and with a 1part PET to 1 part TiO2 concentrate (PET 9663 E0002 from EastmanChemicals) at 2.0% by weight. This blend was then dried in a desiccantdryer at 65° C. for 12 hours.

Also, neat PET(#7352 from Eastman Chemicals) was dried in a desicantdryer at 140 C for 12 hours.

Coextruded cast sheets were extruded using a 2½″ extruder to extrude thePET/PP/TiO2 blend and a 1½″ extruder to extrude the neat PET. The 275°C. meltstreams were fed into a 7 inch film coextrusion die also heatedat 275° C. An ABA film structure with neat PET “A” layers and the blendas the “B” layer were formed in the coextrusion die. As the extrudedsheet emerged from the die, it was cast onto a quenching roll set at 55°C. The PP in the PET matrix dispersed into globules between 10 and 30 umin size during extrusion. The final dimensions of the continuous castsheet were 18 cm wide and 1016 um thick. The “A” layers of neat PET wereeach 356 um thick while the core “B” layer was 304 um thick. The castsheet was then stretched at 110 C first 3.2 times in the X-direction andthen 3.4 times in the Y-direction. The stretched sheet was then Heat Setat 150° C.

During stretching voids were initiated around the particles of PP thatwere dispersed in the cast sheet. These voids grew during stretching andresulted in significant void volume. The resulting the thickness was 112um with the voided “B” layer being 50 μm thick. This film was evaluatedoptically as an unattached diffuser film in combination with a 2 mmthick plate of float glass.

Sample EX-4

PET(#7352 from Eastman Chemicals) was dry blended withPolypropylene(“PP”, Huntsman P4G2Z-159) at 7% by weight and with a 1part PET to 1 part TiO2 concentrate (PET 9663 E0002 from EastmanChemicals) at 0.5% by weight. This blend was then dried in a desiccantdryer at 65° C. for 12 hours.

Cast sheets were extruded using a 2½″ extruder to extrude thePET/PP/TiO2 blend. The 275° C. meltstream was fed into a 7 inch filmextrusion die also heated at 275° C. As the extruded sheet emerged fromthe die, it was cast onto a quenching roll set at 55° C. The PP in thePET matrix dispersed into globules between 10 and 30 um in size duringextrusion. The final dimensions of the continuous cast sheet were 18 cmwide and 1140 um thick. The cast sheet was then stretched at 110° C.first 3.2 times in the X-direction and then 3.4 times in theY-direction. The stretched sheet was then Heat Set at 150° C.

During stretching voids were initiated around the particles of PP thatwere dispersed in the cast sheet. These voids grew during stretching andresulted in significant void volume. The resulting the thickness was 140um. This film was evaluated optically as an unattached diffuser film incombination with a 2 mm thick plate of float glass.

Sample EX-5

PET(#7352 from Eastman Chemicals) was dry blended withPolypropylene(“PP”, Huntsman P4G2Z-159) at 20% by weight but no TiO2concentrate was added. This blend was then dried in a desiccant dryer at65° C. for 12 hours.

Cast sheets were extruded using a 1¼″ extruder to extrude thePET/PP/TiO2 blend. The 275C meltstream was fed into a 7 inch filmextrusion die also heated at 275 C. As the extruded sheet emerged fromthe die, it was cast onto a quenching roll set at 55C. The PP in the PETmatrix dispersed into globules between 10 and 30 um in size duringextrusion. The final dimensions of the continuous cast sheet were 18 cmwide and 300 um thick. The cast sheet was then stretched at 110 C first3.2 times in the X-direction and then 3.4 times in the Y-direction. Thestretched sheet was then Heat Set at 150 C.

During stretching voids were initiated around the particles of PP thatwere dispersed in the cast sheet. These voids grew during stretching andresulted in significant void volume. The resulting the thickness was 51um. This film was evaluated optically as an unattached diffuser film incombination with a 2 mm thick plate of float glass.

Control Sample C1

This comparative sample was the 2.03 mm thick native plate diffusersupplied in the commercial TV used as the test bed for all opticalmeasurements. An Aquos 20″ DBL TV by Sharp Electronics Corporation wasused.

Control Sample C2

This sample was a commercial foam film 100 um thick. It comprised apolyolefin that is foamed by a chemical forming agent. The material ismanufactured by Berwick Industires, Berwick, Pa.

This film was evaluated optically as an unattached diffuser film incombination with a 2 mm thick plate of float glass.

Control Sample C3

This sample was a commercial biaxially oriented voided polypropylene 81um thick. The product name is Polylith GC-1 by Granwell.

This film was evaluated optically as an unattached diffuser film incombination with a 2 mm thick plate of float glass.

Control Sample C4

This sample was a 380 um commercial acrylic foam tape. The product nameis VHB™ No. 4920 by 3M™. This film has been disclosed as a potentialdiffuser film to be attached to a self-supporting substrate. This filmwas evaluated optically by being self-laminated to a 2 mm thick plate offloat glass.

Control Sample C5

This sample was produced by laminating the diffuser film of EX-5 to the2 mm plate of float glass using a clear adhesive transfer tape. The tapeused was a 50 μm thick tape No. 8142 by 3M™. The tape was applied to theglass and then the diffuser film was applied to the tape.

The measurements of brightness comprised an on-axis luminancemeasurement and an on-axis luminance gain calculation. Thesemeasurements along with optical uniformity, for examples EX1-EX5 andcontrol samples C1-C5 were performed on a specially designed LCD-TVexperimental test bed. The test bed apparatus 400, illustratedschematically in FIG. 4 used a commercial backlight unit 410 to mountand illuminate the samples. Either a diffuser plate 402, or acombination of an optically transmissive self-supporting substrate and avoided polymeric optical diffuser film 402 was placed in the backlight.The samples were then measured optically using either of two measuringdevices 420 and 430. A description of the back light unit and themeasuring equipment follows:

Back Light Unit:

-   Aquos 20″ DBL TV by Sharp Electronics Corporation(410 in FIG. 4). 10    CCFL's-   With Diffuser Plate (402 in FIG. 4) of thickness, 2 mm.-   (A 2 mm piece of glass was used in place of the Plate Diffuser as    the optically transmissive self-supporting substrate when measuring    unattached diffuser films, which were placed over the glass, 402 in    FIG. 4.)

Measuring Equipment:

-   1.) ELDIM 160R EZ Contrast conscope—2 mm spot size with a 1.2 mm    distance from sample.(420 in FIG. 4)-   2.) TopCon BM7 colorimeter—1 deg cone, 5 mm spot size, 0.5 meter    distance from sample.(430 in FIG. 4)

The ELDIM 160R EZ Contrast conscope was used to determine the on-axisluminance emitting from the diffuser plate or from the opticallytransmissive self-supporting substrate in combination with an unattacheddiffuser film. On-axis luminance is the intensity of light emittingnormal to the diffuser plate or diffuser film surface. Data was reportedas the luminance in candela per square meter(cd/m²). The on-axisluminance value for all samples was divided by the on-axis luminancevalue for the 2 mm thick native diffuser plate for the backlight todetermine an on-axis luminance gain value.

The TopCon BM7 calorimeter was used to measure optical uniformity forall samples. The 5 mm spot size of the instrument was centered over the#5 of ten CCFL's (spaced nominally 30 mm apart) in the backlight unit tomeasure luminance. This same measurement was made at 5 mm intervals in 4different locations either side of the location directly above CCFL #5,resulting in 9 different measurements nominally centered on CCFL 5. FIG.6 shows results of these measurements over the CCFL's with no diffuserplate or diffuser film. The peak luminance directly over CCFL #5 isobviously a maximum whereas luminance minimums occur at the approximatelocations halfway between CCFL #5 and CCFL #4 on one side and CCFL #6 onthe other side. These minimums are approximately at locations 3 and 8 inFIG. 6, respectively. Optical Uniformity is determined by calculatingthe ratio of the smallest minimum value of luminance in this measurementby the maximum value of luminance made directly over CCFL #5.

Shrinkage testing was done to all diffuser film samples that wereconfigured in an unattached mode to the float glass self-supportingsubstrate. Thermal shrinkage measurements were performed using sampleswith dimensions of approximately 35 mm wide by minimum of approximately6 inches long. Each strip is placed in a punch to obtain a preset 6-inchgauge length. The actual gauge length is measured using a devicecalibrated with a 6-inch invar bar preset to measure 6-inch samples.This length is recorded to 0.0001 inches using a digital micrometer.Once the initial length is determined, samples are placed in an oven atthe prescribed temperature for the necessary time interval (in this casetest condition 85 degrees C. for 24 hours). Samples are then removedfrom the oven and placed in a controlled environment set to 23 degreesC. and 50 % relative humidity for a minimum of approximately 2 hours butgenerally approximately 24 hours. The final sample length is re-measuredusing the same setup used to determine the initial length. The shrinkageis reported in percent using the following equation:

${{Percent}\mspace{14mu} {Linear}\mspace{14mu} {Change}} = \frac{\left( {{{final}\mspace{14mu} {value}} - {{initial}\mspace{14mu} {value}}} \right) \times 100}{{initial}\mspace{14mu} {value}}$

It is noted that the negative (−) sign associated with the shrinkagedenotes direction of the change.

The thickness, optical properties, and shrinkage test results of each ofthe experimental samples and the control samples are summarized in TableI below. In Table 1, each row presents the data for a single sample andthickness of only the diffuser film is shown where both an opticallytransmissive self-supporting substrate and a diffuser film are used incombination for the sample.

TABLE 1 Void V.I. Load × Thickness Tg of Initiator Total Voided On-axisMatrix Loading Thickness Layer Lum. On-axis Optical Shrinkage Sample (°C.) (wt %) (mils/μm) (μm) (cd/m²) Lum.Gain Uniformit (%) EX-1 81 222.4/61 1342 3590 0.905 0.96 0.63 EX-2 81 20 2.1/53 1060 3935 0.992 0.9410.61 EX-3 81 20  4.4/112 1000 3574 0.901 0.969 0.46 (voided-50 μm) EX-481 7  5.5/140 980 3582 0.903 0.965 0.45 EX-5 81 20 2.0/51 1020 41331.042 0.915 0.66 C1 NA NA   80/2030 NA 3967 1 0.952 NA C2 −20 ? 3.8/97 ?3606 0.909 0.947 1.42 C3 −20 ? 3.2/81 ? 3379 0.852 0.969 1.08 C4 NA NA   95/2.41 NA NA C5 NA 20    84/2.13 1020 3351 0.845 0.926 NA

The data in Table 1 shows that the diffuser films of the presentinvention EX-1 thru EX-5 can have optical properties very similar to thecommercial plate diffuser C1. The data also shows that the foamed orvoided films that were evaluated as comparisons C2 and C3, which couldbe unattached to the self-supporting substrate, shrink much more thanthe films of the present invention, EX-1 thru EX-5. These films wouldnot be suitable in this application due to excessive dimensionalinstability. The data also shows that the acrylic foam tape C4 which wasadhered to the glass substrate does not perform well optically, having amuch lower on-axis luminance and lower uniformity than the check platediffuser C1. Comparative sample C5, which is the diffuser film of EX-5laminated to the glass substrate, shows the benefit of not needing tolaminate to the substrate as optical properties can be severely degradedby the addition of an adhesive layer, as shown by the significant dropin on-axis brightness of CS versus EX-5.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described.

On the contrary, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the claims.

The entire contents of the patents and other publications referred to inthis specification are incorporated herein by reference.

PARTS LIST

-   100 direct-lit LC display device-   110 backlight-   112 reflector-   114 light sources-   120 light management layers-   122 diffuser plate-   124 collimating diffuser film-   126 light directing film-   128 reflective polarizer-   130 front LC panel assembly-   132 lower absorbing polarizer-   134 panel plates-   136 LC layer-   138 upper absorbing polarizer-   139 optional layer(s)-   140 LC panel-   150 controller-   200 light management layers-   212 self-supporting substrate-   214 voided polymeric optical diffuser film-   215 bead coated light collimation film-   216 prismatic light directing film-   218 reflective polarizer-   300 light management layers-   312 optically transmissive self-supporting substrate-   314 voided polymeric optical diffuser film-   316 prismatic light directing film-   318 reflective polarizer-   400 test bed apparatus-   402 either voided polymeric optical diffuser film or diffuser plate-   410 backlight unit-   420 measuring device-   430 measuring device

1. A light assembly comprising a light source, an optically transmissiveself-supporting substrate, and coupled with but unattached to saidsubstrate, a voided high Tg semi-crystalline polymeric optical diffuserfilm that shrinks less than 1% as a result of thermal shrinkage testing.2. The light assembly of claim 1, the optical diffuser film comprisingan optical brightener.
 3. The light assembly of claim 1, the opticaldiffuser film comprising polyester as the voided polymer.
 4. The lightassembly of claim 3, the optical diffuser film, wherein the polyestercomprises polyethylene terephthalate, polyethylene naphthalate,polylactic acid, or any of their copolymers.
 5. The light assembly ofclaim 3, the optical diffuser film comprises a polyethyleneterephthalate with a light transmission value greater than 90.5%.
 6. Thelight assembly of claim 1, the optical diffuser film comprising a voidinitiator particle.
 7. The light assembly of claim 6, the opticaldiffuser film wherein said void initiator particle is a polyolefin. 8.The light assembly of claim 7, wherein the optical diffuser filmcomprises polypropylene.
 9. The light assembly of claim 7, wherein saidpolyolefin is present in an amount between 3% and 25% by weight.
 10. Thelight assembly of claim 1, the optical diffuser film wherein saidpolyolefin is present in an amount between 10% and 20% by weight. 11.The light assembly of claim 1, wherein, for the optical diffuser film,the product of the amount of void initiator in weight percent multipliedby the thickness of the voided layer, in aim, is between 750 and 1500.12. The light assembly of claim 11, wherein the product of the amount ofvoid initiator, in weight percent, multiplied by the thickness of thevoided layer, in μm, is between 950 and
 1350. 13. The light assembly ofclaim 2 wherein said optical brightener comprises benzoxazolyll-stilbenecompounds.
 14. The light assembly of claim 2 wherein said opticalbrightener comprises2,2′-(1,2-ethenediyldi-4,1-phenylene)bisbenzoxazole.
 15. The lightassembly of claim 2 wherein optical brightener is present in an amountbetween 0.01 and 0.1 wt %.
 16. The light assembly of claim 2 wherein theoptical brightener is present in an amount between 0.02 and 0.05 wt %.17. The light assembly of claim 1 wherein the optical diffuser filmcomprises titanium dioxide.
 18. The light assembly of claim 17 whereintitanium dioxide is present in an amount between 0.25 and 5 wt %. 19.The light assembly of claim 1 wherein the optical diffuser film ismultilayered.
 20. The light assembly of claim 19, wherein a non-voidedpolymeric layer is adjacent to the optical voided diffuser film on atleast 1 side of said voided film.
 21. The light assembly of claim 1,comprising a structured surface to control the direction of light raystransmitted through the film.
 22. The light assembly of claim 21,wherein said structures are finite curved prismatic structures.
 23. Thelight assembly of claim 21, wherein transparent beads are coated ontosaid structures.
 24. The light assembly of claim 21 wherein an opticalmodifying layer is coated onto said structures.
 25. The light assemblyof claim 1 wherein the on-axis luminance gain is greater than 0.90 andthe localized uniformity is greater than 0.90.
 26. The light assembly ofclaim 1 further comprising an anti-stat coating.
 27. The light assemblyof claim 1, wherein the optical diffuser film thickness is between 1 and10 mils.
 28. The light assembly of claim 1 wherein the thickness of saidfilm is between 2 and 6 mils.
 29. A lighted display including the lightassembly of claim
 1. 30. The display of claim 29 including an LC celllocated on the opposite side of the optical diffuser film from the lightsource.
 31. The display of claim 30 further including other opticallayers between the optical diffuser film and the LC cell.
 32. A processfor displaying an image comprising transmitting light through the filmof claim
 1. 33. A voided high Tg semi-crystalline polymeric opticaldiffuser film that shrinks less than 1% as a result of thermal shrinkagetesting that comprises as the voided polymer a polyethyleneterephthalate with a light transmission value greater than 90.5%.